\documentclass[onecolumn]{article}
% \usepackage{}

\usepackage{amsmath,amssymb,psfrag,color,pbox,subfigure,graphicx,times,textcomp,
url, multirow, rotating}

% type user-defined commands here
\newcommand{\red}{\color{red}}


\begin{document}


\begin{figure}[t!]
\centering
\subfigure{
\includegraphics[width=\linewidth]{plot_textured_single.pdf}
}
\subfigure{
\includegraphics[width=\linewidth]{plot_textured_double.pdf}
}
\subfigure{
\includegraphics[width=\linewidth]{plot_non-textured_single.pdf}
}
\subfigure{
\includegraphics[width=\linewidth]{plot_non-textured_double.pdf}
}
\caption{Grasp results. The stacked-bar plots
show the average distribution of all grasps over all scenes. The stable and slipped grasps
are considered successful grasps, where the object is held in the hand
after lifting. The gray area shows the proportion of scenes where the
methods do not suggest any grasps.}
\label{fig:results_distribution}
\end{figure}

\end{document}